The invention is described below in the context of a particular example of a satellite communication network for which a plurality of users communicate with a satellite communication management station via a return channel. A satellite network of this type may, for example, implement the DVB-S (Digital Video Broadcasting Satellite) standard or developments thereof on the direct broadcasting channel between the management station and the users and the DVB-RCS (Digital Video Broadcasting-Return Channel Satellite) standard or developments thereof on the return channel between the users and the communication management station.
The invention is described below in the particular context of the second-generation development of the DVB-RCS standard, commonly referred to by the acronym DVB-RCS2 and described notably in the document entitled “Digital Video Broadcasting (DVB): Second generation DVB interactive satellite system; part 2: lower layers for satellite standard, DVB document ETSI EN 301 545-2 v1.1.1 (2012-01)”. The invention may also be applied in an equivalent manner to any other communication network in which a problem of sharing resources among users arises in a similar manner.
In a communication network implementing the DVB-RCS standard, access to the communication on the return channel is provided by means of the time/frequency plan of the MF-TDMA (Multi Frequency-Time Division Multiple Access) type defined in advance by the operator and notified to users.
According to this standard, the return channel is subdivided into superframes, which are themselves divided into frames, which are themselves divided into time and frequency intervals. A superframe defines the chosen time/frequency plan, corresponding to a chosen portion of times and frequencies. Within a superframe, the frames and time intervals are classified from the start of the first time interval and the lowest frequency until the end of the last time interval and the highest frequency. Each frame of a superframe defines a portion of the usable time/frequency plan.
Access to the resources is provided by allocating to each user one or more time intervals and one or more carriers within a frame by means of dedicated signalling messages transmitted by the communication management station.
An important aspect for guaranteeing the correct operation of shared access to resources defined by the time/frequency plan relates to the time and frequency synchronization between each user and the communication management station. Users must in fact synchronize their internal clocks with the clock of said station in such a way that they actually communicate in the time and frequency intervals which are allocated to them without interfering with the neighbouring intervals which may be allocated to other users.
To guarantee the synchronization of all users, one solution, compatible with the DVB-RCS standard, is to provide a time/frequency plan arrangement which consists in inserting one or more synchronization intervals at the end of each carrier of a frame. Each terminal transmits a synchronization message in a dedicated interval in order to indicate its presence to the communication management station which recovers this message, performs the synchronization measures and re-transmits information to the user enabling him to perform resynchronization.
The intervals of a frame allocated to synchronization are not usable to communicate and therefore the allocatable communication resources are reduced. The use of one or more reserved synchronization interval(s) per carrier enables correct operation of the network even when it is operating at maximum load. However, this approach results in an over-reservation of resources dedicated to synchronization to the detriment of the resources dedicated to communication when the system is not operating at maximum load.
This problem has all the more impact when the number of carriers per frame is increased, for example due to a degradation of the link budget resulting in a reduction in the bandwidth of a carrier. In such a case, the available resources become scarcer since certain carriers may have a significantly degraded link budget, and therefore a requirement exists to increase the resources dedicated to communication.
More generally, a problem targeted by the present invention relates to the adaptation to the load variations of the network of available resources, between those dedicated to communication and those dedicated to synchronization.
The French patent application published under number FR2874147 is known, which describes a device for the allocation of shared resources of a communication network through allocation of time intervals in a dynamically adaptable time/frequency plan. This application describes the principle of dynamic allocation of carriers by varying the bandwidth of the carriers of a frame as a function notably of the link budget.
However, the problems of optimum sharing of available resources between those dedicated to communication and those dedicated to synchronization are not mentioned.